add a dx parameter for better control of R_smoothing

[lxvm]

This PR adds a dx parameter to the low-level SSP projection interface. The use case for this is typically when the projection is onto a grid of a different resolution than the rho_filtered grid, although dx will still default to the pixel step of the rho_filtered grid. This way, the user has better control of R_smoothing = alg.smoothing_radius * dx without having to pick a dimensionless rescaling of the smoothing_radius. Perhaps if the list of target points is grid-like, dx should default to that spacing, although the choice to make target_points a list of points was made based on the interpolation API from FastInterpolations.jl before the recent 0.4.16 release that allows a GriddedQuery.